11D3 — Double Diode Triode Vacuum Tube

Overview

The 11D3 is an indirectly heated double diode triode vacuum tube designed primarily as a replacement type. It features a 7-pin English base and a top cap connection for the triode grid. The device combines two independent diode sections and a single high-mu triode section within a single glass envelope, sharing a common cathode. This configuration is typically utilized for simultaneous signal detection, automatic volume control (AVC) rectification, and audio-frequency voltage amplification.

The 11D3 operates with a nominal heater voltage of 13.0 V and a heater current of 0.2 A. Its electrical characteristics are closely related to the type 75 double diode triode, but it is distinguished by its specific heater requirements and physical base configuration.

Physical Construction and Pin Connections

The 11D3 is constructed in a glass bulb with a maximum outer diameter of 1-9/16 inches and a maximum overall height of 3-13/16 inches. The bulb features a top cap at the apex, which provides the connection for the triode control grid, minimizing stray capacitance to the other electrodes.

The tube utilizes a 7-pin English base. The pin assignments, viewed from the bottom of the tube base, are as follows:

• Top Cap (TC): Triode Grid (g / G)
• Pin 1: Diode Anode 2 (a''d / D1)
• Pin 2: No Connection
• Pin 3: Diode Anode 1 (a'd / D2)
• Pin 4: Heater (h / H)
• Pin 5: Heater (h / H)
• Pin 6: Cathode (k / C)
• Pin 7: Triode Anode (a / A)

The heater is designed for series or parallel AC/DC operation at 13.0 V, drawing a nominal current of 0.2 A. The calculated power dissipation of the heater is:

$\text{Heater Power} = 13.0\text{ V} \times 0.2\text{ A} = 2.6\text{ W}$

Electrical Characteristics and Operating Points

The triode section of the 11D3 is designed for high-voltage gain. Under typical Class A amplifier operating conditions, the electrical parameters are specified as follows:

The high amplification factor (u = 100) combined with a high anode impedance of 90,000 ohms indicates that the triode section is highly suited for resistance-coupled audio amplifier stages where maximum voltage gain is required. The low typical anode current of 0.4 mA allows the tube to operate efficiently with high-value anode load resistors.

Triode Transfer Curves and Behavior

The transfer characteristics of the 11D3 triode section illustrate the relationship between the grid bias voltage and the resulting anode current across various anode operating potentials:

• At 250 V Anode Voltage: With a grid bias of -2 V, the anode current is positioned at the typical operating point of 0.4 mA. Reducing the negative grid bias causes a sharp increase in anode current; at -1 V grid bias, the current rises to approximately 1.5 mA, and it continues to climb rapidly as the grid voltage approaches 0 V.
• At 200 V Anode Voltage: The curve is shifted to the right. An anode current of 0.4 mA is achieved at a grid bias of approximately -1.5 V.
• At 150 V Anode Voltage: An anode current of approximately 0.35 mA is obtained at a grid bias of -1.2 V.
• At 100 V Anode Voltage: The triode conducts approximately 0.4 mA at a grid bias of -0.5 V.

These curves demonstrate the high sensitivity of the triode grid. The steepness of the curves as grid bias approaches zero volts highlights the high mutual conductance relative to the low operating current.

Comparison with Related Types

The 11D3 is frequently compared to other double diode triodes of similar vintage and application:

Type 75

The electrical characteristics of the 11D3 triode and diode sections are identical to those of the type 75 double diode triode. The primary differences lie in the heater ratings and the physical base configuration, as the 11D3 is designed specifically with a 13.0 V heater and an English base for replacement applications.

Type 11D5

The 11D5 is another double diode triode sharing the same physical dimensions, English base, and pinout configuration as the 11D3. However, it is designed for different circuit requirements:

• Heater Current: The 11D5 draws 0.15 A at 13.0 V, compared to the 0.2 A drawn by the 11D3.
• Amplification Factor: The 11D5 has a lower amplification factor of 40 (compared to 100 for the 11D3).
• Anode Impedance: The 11D5 features a much lower anode impedance of 26,700 ohms (compared to 90,000 ohms for the 11D3).
• Mutual Conductance: The 11D5 has a higher mutual conductance of 1.5 mA/V (compared to 1.10 mA/V for the 11D3).
• Typical Operating Point: At an anode voltage of 250 V and a grid bias of -3 V (using a 750 ohm cathode bias resistor), the 11D5 draws a significantly higher anode current of 3.8 mA, making it more suitable for applications requiring lower output impedance and higher signal-handling capability than the high-gain 11D3.

Technical Summary

The 11D3 is a specialized double diode triode featuring a 7-pin English base, a 13.0 V, 0.2 A heater, and a high-mu triode section. With an amplification factor of 100, an anode impedance of 90,000 ohms, and a transconductance of 1.10 mA/V at its typical operating point of 250 V anode voltage and -2 V grid bias, it is optimized for high-gain voltage amplification. The inclusion of two independent diode anodes sharing a common cathode allows the tube to perform detection and AVC functions alongside audio amplification, making it a versatile replacement valve in classic receiver designs.